Preheat the oven to 325˚F (165˚C). Line 2 muffin pans with 14 – 16 paper liners.
In small heatproof bowl, add the cocoa powder. Pour in the hot water. Add the Nutella. Stir and mix until smooth. Add vanilla extract and mix again. Set aside.
In a large mixing bowl, whisk together the egg yolks, 30g caster sugar and salt. Add the oil and milk, and mix again. Pour in the cocoa mixture and whisk until everything is combined. Sift in both flours and mix well with a spatula.
In a large mixing bowl with an electric mixer fitted with a whisk attachment, add the egg whites. Beat over medium speed until big bubbles appear, add the cream of tartar. Continue to beat until soft peaks form. Add the 65g caster sugar gradually and beat until stiff peaks form. Reduce the mixer speed to lowest and beat for another 10 seconds to minimize air bubbles in the cakes.
Mix 1/3 of the egg whites to the chocolate Nutella mixture to loosen up. Fold in the rest of the whites gently until just combined.
Divide the batter evenly among the cupcake liners, about ¾ filled. Gently tap the muffin pans on a counter top to release air bubbles in the batter.
Bake in the oven for 22 – 25 minutes. Test the cupcakes with a toothpick for doneness. The toothpick should come out clean. If you press the cake lightly with your finger, the cake should spring back. Remove the cupcakes from oven. Transfer the cupcakes to a cooling rack and cool completely.
For the buttercream, in a large mixing bowl with an electric mixer fitted with a paddle attachment, beat the butter until light and fluffy. Add ½ cup sugar and mix on low speed until combined. Taste and add more sugar if desired. Add the Nutella and mix well over the medium speed. Scrape the bowl if needed. Add the vanilla extract and salt. Whip for 30 more seconds. Add the whipping cream and beat for 1 more minutes. Frost and decorate the cupcakes. Right before serving, top each cupcake with a Ferrero Rocher chocolate.